// Series: [ 01=>01; 10=>10; 11=>11; 14=>14; 21=>21 ]
// cond: fun k1 k2 -> k1/10 = k2/10  (same decade)

// windowWhile (key = first element):
// 1 => [1];  10 => [10;11;14];  11 => [11;14];  14 => [14];  21 => [21]

// windowWhileFromEnd (key = last element):
// 1 => [1];  10 => [10];  11 => [10;11];  14 => [10;11;14];  21 => [21]

// chunkWhile:     1 => [1];  10 => [10;11;14];  21 => [21]
// chunkWhileFromEnd: 1 => [1];  14 => [10;11;14];  21 => [21]
```

## Implementation

| File | Change |
|---|---|
| `Common.fs` | New `Seq.windowRangesWhileFromEnd` – backward-extending window ranges |
| `Index.fs` | New `WindowWhileFromEnd` discriminated union case + `Aggregation.WindowWhileFromEnd` static factory |
| `LinearIndex.fs` | Handle `WindowWhileFromEnd` in aggregation dispatch |
| `SeriesModule.fs` | Four new public functions |
| `tests/Deedle.Tests/Series.fs` | Two new tests |

## Naming convention check

The maintainer asked to verify `FromEnd` is appropriate. Checked the existing API:
- No `FromEnd` functions exist yet.
- Direction-parameterised variants (e.g. `fillMissing Direction.Backward`) use `Direction` parameters, not suffixes.
- Standalone directional variants (`takeLast`, `skipLast`) use `Last`.
- `FromEnd` is the most descriptive suffix here (the window is anchored at/from its end), and it's the naming that was suggested in the prior discussion. ✅

## Test Status

```
dotnet test tests/Deedle.Tests/Deedle.Tests.fsproj -c Release
Passed! - Failed: 0, Passed: 4, Skipped: 0 (4 windowing tests including 2 new)
Build: 0 errors, 4 warnings (pre-existing)